1. Field of the Invention
The present invention relates to a method for evaluating reproduction durability reproduction of an optical recording medium capable of recording information by being irradiated with a laser beam, the optical recording medium, and an information-recording/reproducing apparatus.
2. Description of the Related Art
In recent years, the market of the read-only type optical disk such as DVD-ROM and DVD-Video is expanded. The write-once type DVD disk such as DVD−R and DVD+R as well as the rewritable type DVD disk such as DVD-RAM, DVD−RW, and DVD+RW is also introduced into the market. The DVD disk is used as the backup medium for the computer and the image-recording medium with which VTR is to be replaced and then the market of DVD is expanding. On the other hand, the magnetic super resolution technique is used as the technique for realizing the high density in relation to the magneto-optical (MO) disk. In future, the advance of the high density will be further spurred, and the expansion of the market will be expected.
In the case of the optical recording medium as described above, the recording is performed by collecting the laser beam on the recording layer to convert the light energy into the thermal energy so that the characteristics (property and state) of the recording layer is changed, for example, by means of the melting, the decomposition or the removal, or the direction of the magnetization is changed. The reproduction is performed by utilizing the difference in the characteristics between the recorded portion and the non-recorded portion, for example, the difference in the amount of the reflected light or the difference in the direction of the magnetization. Usually, the laser power, which is used during the reproduction, is a low power which does not exert any influence on the characteristics of the recording mark or the direction of the magnetization. However, when the reproduction is repeatedly performed, then the deterioration of the characteristics of the recording mark is caused, or the direction of the magnetization is changed. Therefore, it is extremely important to guarantee the reproduction durability at the reproducing laser power which is at the practical power level as described above. However, for example, in order to guarantee the reproduction for one million times as the reproduction durability, it is necessary that the reproduction should be actually performed repeatedly one million times. Therefore, when the reproducing laser power, which is suitable for the reproduction on the optical recording medium, is determined, and/or when the optical recording medium and/or the information-recording/reproducing apparatus, in which the reproduction durability is to be secured, is developed, then it is necessary that the playback test or the reproduction test should be repeatedly performed a huge number of times.
An acceleration test method, which is disclosed, for example, in Japanese Patent Application Laid-open No. 2003-006941, is hitherto known as a method for evaluating the reproduction durability of the optical recording medium accurately in a short period of time. In this test method, the reproduction durability times (the number of times of reproduction durability) is firstly investigated at a plurality of reproducing powers which is higher than the reproducing power to be used during the ordinary reproduction of information. Subsequently, an obtained result is used to determine the relational expression between the common logarithm of the reproduction durability times and the reproducing power by means of the least square method as a linear approximate expression. The reproduction durability times at an arbitrary reproducing power is estimated from the approximate expression.